FIG. 1 is a cross-section view schematically showing an example of acoustic galvanic isolation device described in French Patent No. 2954014 (incorporated by reference).
This device comprises a silicon substrate 10 coated on its upper surface with a layer 12A of insulating material. Layer 12A has a network 14A of electroacoustic transducers with vibrating membranes formed thereon. Such transducers are often called CMUT, for “Capacitive Micromachined Ultrasonic Transducer”, in the art. Such transducers comprise a conductive layer 16A formed on insulating layer 12A and forming a first electrode common to all transducers. Above conductive layer 16A is formed a layer 17A of a dielectric material. Membranes 18A are defined in layer 17A above cavities 20A. Membranes 18A and opposite cavities 20A have second electrodes 22A formed thereon. One or a plurality of contacts 24A are formed on first electrode 16A. Electrodes 22A are connected to a node 26A.
Symmetrically, the lower surface of substrate 10 comprises elements 12B, 14B, 16B, 17B, 18B, 20B, 22B, 24B, and 26B homologous to elements 12A, 14A, 16A, 17A, 18A, 20A, 22A, 24A, and 26A.
In operation, a D.C. bias voltage is applied between contacts 24A and 26A of first network 14A of transducers and between contacts 24B and 26B of second network 14B of transducers. An A.C. voltage of frequency f0 (input signal) is applied to first transducer network 14A, between contacts 24A and 26A. Such an A.C. voltage creates an oscillation at frequency f0 of membranes 18A of the transducers of network 14A. The generated acoustic ultrasound waves propagate in substrate 10 towards transducer network 14B. The substrate thickness is selected to promote the propagation of acoustic waves of frequency f0. The acoustic waves transmitted by substrate 10 reach the transducers of second network 14B, which causes the vibration of their membrane 18B. This results in the appearing of an A.C. voltage of frequency f0 (output signal) between contacts 24B and 26B.
When the input signal frequency drifts away from frequency f0 for which the thickness of substrate 10 has been selected, a strong attenuation of the amplitude of the output signal can be observed. Thus, such an acoustic galvanic isolation device only operates properly if the frequency of the input signal remains close to a determined frequency f0. This implies associating with the acoustic galvanic isolation device an A.C. input signal generator delivering a frequency which remains close to frequency f0 of optimal operation of the device. The provision of such a generator delivering, with no drift, an accurate frequency results in that this generator should be relatively sophisticated and that its cost is high.
An acoustic galvanic isolation device which can accept an A.C. input signal having its frequency within a relatively wide range is thus needed.